This invention relates to determination of properties of fluids and, in particular, to methods for determination of rheological characteristics of polymer and disperse systems and devices therefor.
The invention can be used to study the rheological properties of a broad class of systems based on polymer materials, adhesives, lacquers, pigments and disperse systems, that is in all fields dealing with investigation and simulation of rheological behavior of various materials.
Methods increasingly used at present to analyze the rheological properties of fluids employ the reaction of systems to mechanical deformation in different conditions in the gap between two elements of a measuring unit by tangentially displacing one element in relation to the other. The most frequently used techniques are continuous shear deformation at a constant rate, harmonic oscillations, and superimposition of harmonic oscillations on the continuous shear deformation.
These conditions can generally be described by the following expression: .gamma.=.gamma..sub.n +.gamma..sub.o .omega.sin.omega.t, where .gamma. is the total deformation rate, .gamma..sub.n is the deformation rate of the steady flow, .gamma..sub.o is the deformation amplitude of periodic deformation, .omega. is the circular frequency of periodic deformation, and t is the elapsed time.
When diagonal components of the complete stress tensor are not equal to zero in viscoelastic systems, it is very often because some extremely objectionable phenomena, from the point of view of the processing technology, such as extrudate bulging, elastic turbulence and disturbance of the flow continuity. To control such phenomena, being the forms in which normal stresses are manifested, eliminates problems in processing of viscoelastic systems, such as improvement of quality or increasing the output capacity of equipment. One of the ways to deal with the problem may be the application of forces perpendicular to the flow direction, since it provides an opportunity to control the components of the complete stress tensor in the process of mechanical deformation of investigated systems under the above conditions.
Known in the art is a method for determination of dynamic properties of viscoelastic fluids (cf., for example, Vibratsionnaya Viskoziemtria, Collection of articles published by the Siberian branch of Institut Teplofiziki AN SSSR, Novosibirsk, 1976, A. I. Isayev, A. K. Kulapov, G. V. Vinogradov, Instrument for Determination of Dynamic Characteristics of Viscoelastic Fluids, pp. 91-106) comprising the steps of placing the investigated fluid in the gap between two coaxial cylinders, mechanically deforming said fluid by harmonic oscillations through tangential displacement of one of the cylinders in relation to the other, and measuring the applied deformation. Simultaneously, to be measured is the angular displacement transmitted through the investigated fluid to the other cylinder in relation to which the first cylinder makes its tangential motions. The two quantities obtained serve to determine the rheological properties of the fluid: the complex dynamic shear modulus and its components, such as the modulus of elasticity and the modulus of losses and tangential stress.
Also known in the art is a device realizing said method (cf., for example, Vibratsionnaya Viskozimetriya, collection of articles published by the Siberian branch of Institut Teplofiziki of the Academy of Sciences of the USSR, Novosibirsk, 1976, A. I. Isayev, A. K. Kulapov, G. V. Vinogradov, Instrument for Determination of Dynamic Characteristics of Viscoelastic Fluids, pp. 91 106) comprising a measuring unit composed of two coaxial cylinders arranged with a gap wherein the investigated fluid is placed, the first cylinder being secured to the body of the device, while the second cylinder is rigidly connected with an output shaft of a drive comprising a motor system equipped with reducing and link gears in order to transform the motor rotational motion into the oscillations of the output shaft of the drive, and a transducer to convert the second cylinder angular displacements into electrical signals, which is placed on the output shaft of the drive and is electrically coupled to a recorder. The first cylinder is placed on the shaft so that it is capable of torsional vibrations in relation to said shaft whose one end is rigidly secured to the instrument body and which mounts a transducer of angular displacements of this cylinder into electrical signals, said transducer being also coupled to a recorder.
This method for determination of dynamic characteristics of viscoelastic fluids and the device realizing it have certain limitations. They can determine rheological characteristics of fluids only in harmonic oscillation conditions, since the device has only one fixed position of the motor system in relation to the output shaft of the drive, wherein this output shaft is set into oscillatory motion.
Known in the art is a method for determination of rheological characteristics of polymer and disperse systems (cf., for example, Technical Description and Operational Instructions 1939-00-00-TO of Vibroreometr VR-74. Spetsialnoye Konstruktorskoye Byuro Instituta neftekhimicheskogo sinteza im. A. V. Topcieva AN SSSR, Moscow, 1975) comprising the steps of placing a polymer or disperse system to be studied in a gap between two coaxial cylinders, subjecting said system to mechanical deformation by tangentially displacing one of the cylinders in relation to the other under the conditions of continuous shear deformation at a constant rate, of harmonic oscillations, and of superimposition of harmonic oscillations on continuous shear deformation, and measuring said deformation.
Concurrently, measurements are made of the angular displacement transmitted through the investigated system to the cylinder in relation to which the other cylinder makes tangential displacements. The two quantities are the basis for determination of the rheological characteristics: the complex dynamic shear modulus and its components--the modulus of elasticity and the modulus of losses and shear stress.
Also known is a device to realize the method for determination of rheological characteristics of polymer and disperse systems (cf., for example, Technical Description and Operational Instructions 19139-00-00-TO of Vibroreometr VR-74, published by Spetsialnoye konstruktorskoye byuro Instituta neftekhimicheskogo sinteza im. A. V. Topchieva AN SSSR, Moscow, 1975), comprising a measuring unit composed of two coaxial cylinders made of a current-conducting material and arranged with a gap wherein the investigated viscoelastic system is placed, the first cylinder being secured to the case of the device, while the second is rigidly connected to the output shaft of the drive, which transforms the signal fed from a control unit electrically coupled thereto into the angular displacement of said cylinder, and a transducer to convert the angular displacement of the second cylinder into an electrical signal, said transducer being positioned on the output shaft of the servo drive and coupled to a recorder. The first cylinder of the measuring unit is fixed on the shaft so that it is capable of making torsional vibrations in relation to said shaft whose one end is secured in the case of the device. The shaft also carries the transducer of the angular displacement of this cylinder into an electrical signal, which is coupled to a recorder.
The first cylinder of the measuring unit is held radially by an air bearing which is used to reduce friction when measuring the angular displacement of the first cylinder. The air bearing requires additional equipment items, such as a compressor, air filters, an air pressure regulator and stabilizer. The external cylinder has one butt end open and the investigated system, being placed in the gap between the coaxial cylinders, has one surface thereof in contact with the air.
The method employed to determine rheological characteristics is based on measuring the deformation and angular displacement of the first cylinder of the measuring unit and cannot, therefore, be used to determine normal stresses.
The air exposed surface of the investigated system can, in the process of deformation, lead to a secondary flow in the axial direction and subsequent explusion of the system from the gap.
The device realizing this method is not suitable for measuring normal stresses produced in the process of deformation. Moreover, the measuring unit composed of two coaxial cylinders indeed provides a highly uniform deformation field but gives no means to apply additional forces to the system in the direction perpendicular to the flow by mechanically displacing one cylinder radially in relation to the other cylinder.
In addition, the use of additional equipment supporting the operation of the air bearing makes the device more complicated.